1. Field of the Invention
The present invention relates generally to a multi-antenna system, and in particular, to an apparatus and method for selecting users and the number of transmission streams for them in order to realize Spatial Division Multiple Access (SDMA) with the aim to increase capacity in a multi-user multi-antenna system.
2. Description of the Related Art
Along with the recent rapid increase in the use of multimedia service, demands for high-speed transmission of large amount of data on radio channels have been increasing. Accordingly, studies have been made on transmission of a large amount of data on mobile channels and radio channels. One of the studies is focusing on a multi-antenna system such as a Multiple-Input Multiple-Output (MIMO) scheme.
A MIMO system uses multiple antennas at both the transmitter and receiver. Compared to a single-antenna system, the MIMO system can increase transmission capacity without using additional frequency or increasing transmission power. It also services multiple users.
The multi-user MIMO system can maximize capacity by Dirty Paper Coding (DPC). Due to its implementation complexity, however, DPC makes it difficult to implement in the MIMO system. That's why the MIMO system adopts SDMA that increases capacity by allowing a plurality of users simultaneously access in time and frequency.
Implementation of SDMA can be considered in two ways. One is to select SDMA users according to the capacities of multiple users for a given number of transmission streams per SDMA user and the other is to determine the number of transmission streams for each of given SDMA users.
In the former case, because the number of transmission streams per user is fixed, the diversity effect resulting from selecting transmission streams for each user is not achieved. In the latter case, because SDMA user groups are fixed, multi-user diversity is not achieved.
To avoid the above problem encountered with the MIMO-SDMA system, the MIMO system can implement SDMA by selecting SDMA users and determining the number of transmission streams for each of the SDMA users simultaneously.
To do so, the MIMO system compares the performance metrics of all possible user combinations with those of all possible stream combinations available to the user combinations. As a consequence, system complexity exponentially increases with the number of total users and the number of available streams.